I. Field of the Invention
The present invention relates to the field of digital communications. More particularly, the present invention relates to a method and apparatus for delivering data within noisy and unpredictable communication environments.
II. Description of the Related Art
In noisy or unpredictable data transmission environments it is often useful to provide a degree of redundancy in the delivery of information or data between two or more systems. For example, in some wireless cellular telephone systems, information from a subscriber unit may be delivered through multiple base transceiver stations, or "cell sites", before being sent to the receiving communication system. The state of being in communication with multiple base transceiver stations within the context of a cellular telephone system is referred to as "soft hand-off." By providing multiple deliveries of information from the subscriber unit to the receiving communication system the probability of one instance of the information arriving successfully is increased, even as the position of the subscriber unit and surrounding environmental conditions change. In general, the redundant delivery of data between two systems is performed via multiple routes as in the soft hand-off situation, however, the multiple deliveries of data can also be performed over the same route at different times.
At some point in the course of redundant delivery of data a single instance of the data must be generated using the multiple instances of data available. One method of generating such a single instance of data is to combine or add the multiple instances together. Generating a single instance of the data in this way is especially useful in the context of a system incorporating the use of RF transmissions because combining these signals after they are transmitted can further enhance the accuracy of the data produced upon further processing. One drawback of generating a single instance of data in this way, however, is that once a single instance of data has been generated, additional data from other instances of the signal can not be added. When multiple signals are received at remote locations, however, as is the case with two separate base transceiver stations during soft hand-off, transmission of the signals in an unprocessed form to a location where they can be combined may be impractical or too costly. Thus, a method other than combining for generating a single instance of data from multiple instances is desired.
It is also useful within noisy and unpredictable transmission environments to incorporate the use of various error detection methods that allow the system receiving data to determine if any errors were introduced into the data during transmission. One method of error detection is to calculate a cyclical redundancy check (CRC) sum for a given segment of data, and to transmit the CRC along with that data. When the data is received, it can be checked against the CRC to verify its accuracy. While very effective, the use of a CRC check sum requires additional data be transmitted along with the original information and, therefore, either reduces the amount of information that can be transmitted, or requires additional bandwidth.
Another method of error detection is the calculation of a "Yamamoto metric" in conjunction with Viterbi encoded data. The calculation of a Yamamoto metric involves tracking the difference between the log likelihood of the most likely result and the next most likely result in a Viterbi decoding procedure. Viterbi decoding, well known in the art, involves performing multiple decodings of data and selecting the particular decoding that has the greatest log likelihood of accuracy. If the difference between the decoding that is most likely to be accurate and the next most likely remains greater than a predetermined amount there is greater confidence in the accuracy of the data, and the Yamamoto metric is set to success indicating an increased likelihood of accuracy. If the difference is less than the predetermined amount the Yamamoto metric is set to fail indicating the data is less likely to be accurate. The exact value of that predetermined amount must be determined via experimentation and the basis of the particular type of communication being performed. The Yamamoto metric is used and described in H. Yamamoto & K. Itoh, "Viterbi Decoding Algorithm for Convolutional Code with Repeat Request", IEEE Transactions on Information Theory, Vol. IT-26, No. 5, September 1980.
Still another method of error detection is the calculation of a re-encoded signal error rate (SER). The calculation of an SER involves re-encoding Viterbi decoded data and comparing this data with the encoded data originally received. The SER is usually a multi-bit value and it can be compared with other SER values calculated in response to other data transmissions to determine relative probability of error between the two transmissions. Of the three error detection methods discussed, CRC is the most likely to detect errors introduced during transmission and the SER is least likely, with the Yamamoto metric providing an intermediate probability of detecting errors.
The three error detection methods described above are each designed to operate on a single instance of data. In wireless cellular telephone systems and other systems incorporating the use of redundant data delivery, however, multiple instances of data are available for enhancing the accuracy of the information being delivered. These multiple instances of data provide an opportunity to increase the accuracy of the information ultimately produced that is not exploited by the above described error detection methods. Therefore, a method and system for generating a single instance of data from multiple instances of that data that incorporates the methods of error detection described above, and that exploits the availability of the multiple instances of data available in a communication system using redundant data delivery technology would be highly desirable.